There has been growing demand for improving the recording density of a magnetic disk device, which is a kind of information storage system mounted in a computer, with an increase in the amount of information which should be processed. In order to improve the recording density of a magnetic disk device, the size of the magnetic recording has been pushed finer. Thus, there have been attempts to make the distance between a magnetic disk and a magnetic head smaller, to make the grain size of the magnetic grains included in a magnetic recording layer of a magnetic disk finer, and to increase the magnetic coercivity (anisotropic field) of the magnetic disk.
However, there is a physical limitation to the increase in the magnetic field strength of a magnetic recording head, and this becomes a factor which limits an improvement in the magnetic recording density. Although, it is necessary for a high density recording to make the crystal grains of a magnetic disk finer in terms of noise reduction, a problem arises in that the magnetic grains become thermally unstable. Therefore, in order to make the crystal grains finer and further, to ensure the thermal stability, the magnetic anisotropic energy has to be increased. An increase in the magnetic anisotropic energy, that is, an increase in the anisotropic field (magnetic coercivity) requires an increase in the head field strength for recording. However, because of an upper limitation of the saturated magnetization of a magnetic pole material utilized for a recording head and a limitation for downsizing the distance between the magnetic disk and the magnetic head, it is difficult to increase the anisotropic field with an increase in the recording density and to increase the magnetic field strength from the magnetic head.
In order to solve the aforementioned problems, a thermally assisted magnetic recording technology which combines an optical recording technology with magnetic recording technology has been proposed and given attention. For instance, there is a technology described in InterMag 2000 HA-04 and HA-06. A magnetic read/write head used herein is one to which a mechanism for heating a disk has been added. During recording, the disk is heated by generation of an applied magnetic field, thereby, the disk magnetic coercivity is decreased. As a result, recording becomes easily performed even by a disk with high magnetic coercivity where recording was hardly performed in a magnetic head of the prior art because of insufficient recording magnetic field strength. Herein, a heat source using light utilizes a method where a laser beam is focused by a lens used for an optical recording. However, there is a limit to the increase in speed and recording density of a magnetic disk device because it is necessary to heat and cool a minute heating region rapidly. As a method to solve this, a method for generating a near field light has been proposed in Optics Japan 2002 Extended Abstracts, 3pA6 (2000), and Japanese Patent Application No. 2003-45004, and research has been carried out. There is a significant relationship between the width of the thermal power distribution and the speed of thermal diffusion and for high speed heating and cooling it is necessary to make the size for heating the magnetic disk smaller than the minimum spot size which can be focused by a lens. Moreover, as a magnetic disk configuration which is preferable for rapid cooling, it is effective to provide a heat sink layer composed of a material having a large thermal conductivity and to enhance the thermal diffusion. An MR (magnetoresistive type) head, which is used in a magnetic recording of the prior art, is used for reproducing. The aforementioned recording method is called a thermally assisted magnetic recording.
In a thermally assisted magnetic recording, thermal decay of magnetization which occurs right after recording is perceived as a problem for improvement of the recording density. There is a concern that thermal fluctuation will be accelerated by heating, that magnetization direction and magnetic domains recorded once will be deleted, and that there will be a failure of high density magnetic recording. As measures for this event, an approach is proposed in Japanese Patent Application No. 2005-310369 where a magnetic recording layer is combined with an exchange magnetic layer and the magnetic coercivity of the magnetic disk is rapidly increased in the cooling process to suppress the magnetization reversal by thermal fluctuation, that is, an introduction of a layer which has a function of switching of the exchange coupling force against the temperature. An approach is also proposed in Japanese Patent Publication No. 2002-175602 where the magnetization reversal by thermal fluctuation is suppressed by utilizing a magnetic field from a magnetic field generation mechanism from outside the magnetic recording head.
As described above, in a thermally assisted magnetic recording, a method for suppressing the magnetization reversal caused by thermal fluctuation has been conventionally proposed. However, it is difficult to avoid the effect of thermal fluctuation with an increase in the speed and recording density.